1. Field of the Invention
This invention pertains generally to controlled rectifier bridges for converting alternating current (AC) to direct current (DC), and, in particular, to three-phase semiconductor controlled rectifier (SCR) bridges for providing excitation voltage to generators. The invention also pertains to controlled rectifier bridge systems.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs). When the generator is on-line, generator field excitation is provided thereto. However, certain faults in the output power circuit of the generator may disturb the voltage fed into the excitation control system, thereby causing the excitation to the generator to shut off. For example, disturbances in the input voltage to the phase lock loop (PLL) of the control system may be caused by faults, such as single line-to-ground faults, line-to-line faults and three-phase faults that occur in the power circuit when the generator's circuit breaker contacts are closed.
When starting a field excitation control system, the AC input voltage to the controlled rectifier bridge is very low or non-existent. This is also the case during certain faults. Prior proposals have phase-controlled the bridge in order to obtain the desired output and, thus, have required some measurable AC input voltage to start the system. Typically, this AC input voltage was used as an input to a PLL or other timing circuit in order to maintain the requisite timing for firing the cells (e.g., SCRs) of the bridge. Accordingly, there is room for improvement.